Electronic imaging devices are used in a wide range of applications and are steadily becoming less expensive and simpler to use. Electronic imaging devices, such as digital cameras, typically convert light to electrical signals using a two-dimensional photodetector. The photodetector contains a two-dimensional array of thousands or even millions of light-sensitive cells, each capturing one picture element (or pixel) of an image. As an image is focused onto the photodetector, an electrical charge builds up in the photodetector cells. The magnitude of the electrical charge corresponds to the intensity of the image lightxe2x80x94brighter light generates a larger electrical charge. Thus, the image light focused on the photodetector generates a pattern of varying electrical charges across the photodetector. The magnitudes of the electrical charges across the photodetector are analyzed and stored in the electronic imaging device as an electronic representation of the image. In a digital imaging device such as a digital camera, the magnitude of the electrical charges is converted into a number in an analog to digital (A/D) converter.
Several types of two-dimensional photodetectors are commonly used in electronic imaging devices, including charge-coupled devices (CCDs) and complementary metal-oxide semiconductor (CMOS) sensors. The electrical charges in a CCD are read by shifting or moving them in a series across the photodetector and reading them one by one at an output. The electrical charges in the CCD are amplified if needed and read by an external A/D converter. CCD photodetectors provide high-quality images because they are not very susceptible to noise, but they consume a great deal of power and must be read in serial fashion, bit by bit. CMOS sensors are generally more susceptible to noise than CCDs but are much less expensive because they can be manufactured on standard silicon integrated circuit production lines. Each cell in a typical CMOS sensor includes several transistors to amplify the electrical charge internally and move it across traditional wires. Each row is generally accessible, starting at row zero and providing input pulses to shift to the desired row. A clock input is then used to access the desired pixels in the selected row. Thus, desired image data can be accessed without shifting the entire image to the output.
Several main focusing techniques exist for electronic imaging devices having photodetectors such as CMOS sensors or CCDs, including active and passive focusing. Active focusing involves shining infrared light from the electronic imaging device onto a focus object. A photosensor in the electronic imaging device receives infrared light reflected from the focus object and compares it with the transmitted infrared light to determine the distance from the electronic imaging device to the focus object. A lens in the electronic imaging device can then be set to the proper focus position according to that distance. Active focusing can be very fast, but it increases the cost of an electronic imaging device and it does not work well with some types of focus objects.
Passive focusing involves capturing consecutive images while adjusting the lens in the electronic imaging device and comparing the images to determine when the electronic imaging device is properly focused. For example, the electronic imaging device may search for the focus position that results in the maximum intensity difference between adjacent pixels in an image. Passive focusing is well suited for inexpensive imaging devices because it does not require additional expensive and complex components. However, conventional passive focusing is substantially slower than active focusing. This situation adds to the already long image capture delay in most electronic imaging devices.
An embodiment of the invention decreases the time required to passively focus an electronic imaging device by effectively decreasing the field of focus used to focus the electronic imaging device. During the passive focusing process, images are repeatedly captured at different focus settings and the image data is analyzed to determine which of the focus settings used is best, using any suitable technique such as searching for maximum contrast.
During this passive focusing process, images are captured at different focus settings using a randomly accessible photodetector such as a CMOS sensor in the electronic imaging device. As the passive focusing process is being performed, the images are analyzed to identify at least one region in the image which is most affected by focus changes. Subsequent images captured for the passive focusing process are captured from the most affected region rather than from the entire randomly accessible photodetector. (Alternatively, a sequentially accessible photodetector such as a CCD may be used, with unwanted image data discarded.) The process of identifying changing regions may be performed recursively during the focusing process, selecting successively smaller regions to sample from the previous larger regions, thereby reducing the amount of data processed and reducing the time required to passively focus the electronic imaging device.